OSLO  The Gulf Stream carrying warm water to the North Atlantic slowed about 10 percent in the Little Ice Age from 1200 to 1850, said a U.S. study published on Wednesday that may give clues to the effects of modern global warming.

The report suggested natural shifts in the current might have affected climate in past centuries.

The Gulf Stream carries waters from the Gulf of Mexico and boosts temperatures in northern Europe by between 5 and 10 degrees Celsius (9 and 18 degrees Fahrenheit) compared with similar latitudes in Siberia.

Driven by winds and the earth's rotation and pushing heat towards the poles, the warmth it carries is equivalent to 2,000 times the electricity generating capacity of the United States.

"The strength of the Gulf Stream was about 10 percent weaker during the Little Ice Age," David Lund, of the Massachusetts Institute of Technology/Woods Hole Oceanographic Institution, told Reuters.

He and two colleagues studied sediment cores off Florida and the Bahamas, and found evidence of a weaker flow that may have contributed to the Little Ice Age from about 1200-1850, when Alpine glaciers grew and London's Thames River froze.

"The possibility of abrupt changes in Gulf Stream heat transport is one of the key uncertainties in predictions of climate change for the coming centuries," the scientists wrote in the journal Nature.

Lund said the slowing of the Gulf Stream in the Little Ice Age might have been caused by shifts in winds in the Atlantic. Causes of that shift were unknown with some researchers blaming changes in the sun's output.

Lund said the "jury is still out" on the future effects of global warming on the Gulf Stream. Many scientists say human emissions of greenhouse gases, mostly from burning fossil fuels, are heating the planet and could disrupt the current.

Some scientists speculate that a melting of the Greenland ice cap, triggered by global warming, could flood the North Atlantic with extra fresh water and slow the Gulf Stream, chilling Europe and parts of North America.

The Gulf Stream carries about 31 Sverdrups of water per second into the North Atlantic -- a Sverdrup is an imaginary cube with sides 100 metres (110 yards) long going past a fixed point. That makes the current 2,000 times the flow of the Mississippi River.

In the Little Ice Age, the current slowed by about 3 Sverdrups, the study showed. And at the height of the last Ice Age, about 20,000 years ago, other studies have estimated that the flow slowed by 10-15 Sverdrups.

Lund said most studies of a possible slowing of the Gulf Stream linked to global warming focused on a cooling of Europe and North America.

However, a shift of winds linked to the current could cause disruption to rainfall in Africa or Central America.